In recent years, thin electronic devices, such as smartphones and tablets, have been widely used. There is an increasing demand for a reduction in the thickness of nonaqueous electrolyte secondary batteries used as power supplies to drive them. Nonaqueous electrolyte secondary batteries in which electrode assemblies are arranged in prismatic cans or pouches, each of the electrode assemblies including a positive electrode plate, a negative electrode plate, and a separator, the positive electrode plate and the negative electrode plate being wound with the separator provided therebetween, have been widely used as thin batteries.
A typical nonaqueous electrolyte secondary battery is designed in such a manner that the capacity ratio of a negative electrode to a positive electrode is higher than 1. The capacity ratio is calculated on the basis of the capacity per unit area of each of the positive electrode and the negative electrode. Electrode plates and a separator of a flat wound electrode assembly each include two straight portions and two corner portions connecting the two straight portions in a section in a direction perpendicular to a winding axis of the electrode assembly. In the straight portions, the area of a positive electrode plate is equal to the area of a negative electrode plate facing the positive electrode plate with the separator provided therebetween, and thus, the capacity ratio is matched to its design value. In the corner portions, however, the electrode plate disposed on a more inner peripheral side of the electrode assembly has a smaller curvature radius, and thus, the area of a positive electrode plate is not equal to the area of a negative electrode plate facing the positive electrode plate with the separator provided therebetween. This causes a deviation of the capacity ratio in each of the corner portions from the design value. The amount of the deviation increases on the more inner peripheral side. In particular, in a portion where the positive electrode plate and the negative electrode plate face each other, the capacity ratio in a portion where the negative electrode plate is disposed on an inner peripheral side of the electrode assembly is lower than the design value, thus leading to a high state of charge of the negative electrode.
A flat wound electrode is produced by winding a predetermined amount of a separator on a winding core portion, then inserting a positive electrode plate and a negative electrode plate into the winding core portion, and winding them. As described above, the predetermined amount of the separator is disposed in the innermost peripheral portion of an electrode assembly. This prevents the winding of the negative electrode plate in an excessively small curvature radius in corner portions of the electrode assembly. However, in the case where an electrode assembly is pressed under heavy load or where the amount of the separator wound in a leading portion is reduced in order to further reduce the thickness of a battery, the curvature radius of the negative electrode plate is reduced in corner portions of the innermost peripheral portion, leading to a high state of charge of the negative electrode in the corner portions. This can cause the negative electrode plate to swell or deform to swell the battery.
Patent Literature 1 discloses a secondary battery in which a positive electrode active material is applied so as to have a small thickness and a negative electrode active material is applied so as to have a large thickness in corner portions of a flat wound electrode assembly in such a manner that the capacity ratio is not less than 1 even in the corner portions. However, it is difficult to change the thickness of part of an electrode plate and locate positions of the electrode plate corresponding to the corner portions of the electrode assembly prior to the production of the electrode plate. It is thus difficult to use the technique described in Patent Literature 1 as a mass production technique.
Patent Literature 2 discloses a secondary battery in which the thickness of a separator disposed on an outer peripheral side of a negative electrode plate is larger than the thickness of a separator disposed on an inner side of the negative electrode plate. The use of this structure prevents an internal short-circuit even if a region of an electrode assembly having a capacity ratio less than 1 is formed to precipitate lithium. However, although the thickness of the separator disposed on the outer peripheral side of the negative electrode plate is increased, the curvature radius of the negative electrode plate in corner portions is not changed. Instead, the curvature radius of a positive electrode plate disposed on the outer peripheral side of the separator having an increased thickness is increased to increase the area of the positive electrode plate facing the negative electrode plate, compared with the negative electrode plate disposed on the inner peripheral side. That is, the technique described in Patent Literature 2 further increases the deviation of the capacity ratio in the corner portions and thus is not inhibit the swelling of the battery due to the swelling or deformation of the negative electrode plate.
Patent Literature 3 discloses a secondary battery in which an insulating tape is bonded to the innermost peripheral side portion of portions where a positive electrode plate and a negative electrode plate face to each other in corner portions of an electrode assembly in order that the innermost peripheral side portion may not participate in charge-discharge reactions. When the insulating tape is bonded to the portion where the positive electrode plate and the negative electrode plate face to each other in the corner portion to prevent the occurrence of the charge-discharge reactions, the problem of an increase in the state of charge of a negative electrode is avoided. In the technique described in Patent Literature 3, however, the capacity reduction of a battery is inevitable, thereby failing to obtain a high-capacity battery.